In this work, a device model for Cu₂ZnSnS₄ (CZTS) solar cell with certified world record efficiency is presented. A study of the most important loss mechanisms and its effect on solar cell performance was carried out. The trap-assisted tunneling and CdS/CZTS interface recombination are introduced as the most important loss mechanisms. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations (quantum efficiency, efficiency, J_sc, FF, and V_oc) reported under normal operating conditions. Finally, a discussion about a further solar cell efficiency improvement is addressed.